A submersible autonomous sensor for spectrophotometric pH measurements of natural waters

An autonomous sensor for long-term in situ measurements of the pH of natural waters is described. The system is based upon spectrophotometric measurements of a mixture of sample and sulfonephthalein indicator. A simple plumbing design, using a small, low-power solenoid pump and valve, avoids the need for quantitative addition of indicator. A approximately 50-microL slug of indicator is pulled into the sample stream by the pump, and subsequent pumping and mixing results in a section of indicator and sample where absorbance measurements can be made. The design also permits direct determination of the indicator pH perturbation. Absorbances are recorded at three wavelengths (439, 579, and 735 nm) using a custom-built 1.7-cm path length fiber-optic flow cell. Solution blanks are obtained by periodically flushing the cell with sample. Field tests were performed in a local river over an 8-day period. The in situ accuracy, based on comparison with laboratory spectrophotometric pH measurements, was -0.003 pH unit (n = 16), similar to the measurement precision. No drift was observed during the 8-day period. The absorbance ratio used to calculate pH, in combination with a simple and robust optical design, imparts an inherent stability not achievable with conventional potentiometric methods, making the design feasible for long-term autonomous pH measurements.     

Tracer monitored titrations: measurement of total alkalinity

We introduce a new titration methodology, tracer monitored titration (TMT), in which analyses are free of volumetric and gravimetric measurements and insensitive to pump precision and reproducibility. Spectrophotometric monitoring of titrant dilution, rather than volume increment, lays the burden of analytical performance solely on the spectrophotometer. In the method described here, the titrant is a standardized mixture of acid-base indicator and strong acid. Dilution of a pulse of titrant in a titration vessel is tracked using the total indicator concentration measured spectrophotometrically. The concentrations of reacted and unreacted indicator species, derived from Beer's law, are used to calculate the relative proportions of titrant and sample in addition to the equilibrium position (pH) of the titration mixture. Because the method does not require volumetric or gravimetric additions of titrant, simple low-precision pumps can be used. Here, we demonstrate application of TMT for analysis of total alkalinity (A(T)). High-precision, high-accuracy seawater A(T) measurements are crucial for understanding, for example, the marine CaCO3 budget and saturation state, anthropogenic CO2 penetration into the oceans, calcareous phytoplankton blooms, and coral reef dynamics. We present data from 286 titrations on three types of total alkalinity standards: Na2CO3 in 0.7 mol kg x soln(-1) NaCl, NaOH in 0.7 mol kg x soln(-1) NaCl, and a seawater Certified Reference Material (CRM). Based on Na2CO3 standards, the accuracy and precision are +/-0.2 and +/-0.1% (4 and 2 micromol kg x soln(-1) for A(T) approximately 2100-2500 micromol kg x soln(-1), n = 242), using low-precision solenoid pumps to introduce sample and titrant. Similar accuracy and precision were found for analyses run 42 days after the initial experiments. Excellent performance is achieved by optimizing the spectrophotometric detection system and relying upon basic chemical thermodynamics for calculating the equivalence point. Although applied to acid-base titrations in this paper, the approach should be generally applicable to other types of titrations.     

Evaluation of indicator-based pH measurements for freshwater over a wide range of buffer intensities

Two different sulfonephthalein indicators, cresol red (CR) with a pKa of approximately 8.3 and bromothymol blue (BTB) with pKa of approximately 7.4, were tested for an analysis of freshwater over a broad range of pH and total alkalinity values. Measurements from an autonomous sensor system using a 1 cm optical path length were compared to those using a 10 cm path length on a benchtop spectrophotometer. The indicator pH perturbation was quantified with a thermodynamic model and nonlinear least-squares analysis. The laboratory study found that the perturbation-corrected pH differed between the 1 cm (large indicator perturbation) and 10 cm (small indicator perturbation) optical path length measurements from -0.017 to +0.15 with a median of +0.0041 pH units for CR and from -0.015 to +0.026 with a median of -0.0008 pH units for BTB. Precision was +0.0005-0.013 and +0.0001-0.0027 pH units for the 1 and 10-cm-path-length measurements, respectively. The autonomous sensor was deployed for 14 days in a local creek. Simultaneous glass pH electrode measurements had a large negative and drifting offset (-0.15 to -0.40 pH units) compared to the indicator-based measurements. This study is the first in situ comparison between potentiometric and spectrophotometric pH methods in a freshwater system.     

Modeling the effects of combined behavioral and pharmacological treatment on cigarette smoking: behavioral-economic analyses

Effects of different response requirements, response-independent cigarette puffs, and an alternative nondrug reinforcer on cigarette smoking were assessed in 2 experiments. The response requirement to obtain 2 puffs on a cigarette was manipulated while various numbers of response-independent puffs were provided (0, 6, or 12 in Experiment 1; 0 or 12 in Experiment 2). In Experiment 2, effects of response-dependent money ($0.25) on smoking were assessed within subjects. Response-dependent puff consumption decreased as price increased and as the number of response-independent puffs increased. Concurrently available money also decreased response-dependent smoking. The largest decrease in smoking occurred when puffs were at the highest price and when response-independent puffs and response-dependent money were both concurrently available. Findings suggest that combined pharmacological and behavioral interventions produce the greatest reductions in smoking.     

Biogeochemical controls on Diel cycling of stable isotopes of dissolved O2 and dissolved inorganic carbon in the Big Hole River, Montana

Rivers with high biological productivity typically show substantial increases in pH and dissolved oxygen (DO) concentration during the day and decreases at night, in response to changes in the relative rates of aquatic photosynthesis and respiration. These changes, coupled with temperature variations, may impart diel (24-h) fluctuations in the concentration of trace metals, nutrients, and other chemical species. A better understanding of diel processes in rivers is needed and will lead to improved methods of data collection for both monitoring and research purposes. Previous studies have used stable isotopes of dissolved oxygen (DO) and dissolved inorganic carbon (DIC) as tracers of geochemical and biological processes in streams, lakes, and marine systems. Although seasonal variation in 6180 of DO in rivers and lakes has been documented, no study has investigated diel changes in this parameter. Here, we demonstrate large (up to 13%o) cycles in delta18O-DO for two late summer sampling periods in the Big Hole River of southwest Montana and illustrate that these changes are correlated to variations in the DO concentration, the C-isotopic composition of DIC, and the primary productivity of the system. The magnitude of the diel cycle in delta18O-DO was greater in August versus September because of the longer photoperiod and warmer water temperatures. This study provides another biogeochemical tool for investigating the O2 and C budgets in rivers and may also be applicable to lake and groundwater systems.     

Universal tracer monitored titrations

Titrations, while primarily known as the chemical rite of passage for fledgling science students, are still widely used for chemical analysis. With its many years of existence and improvement, the method would seem an unlikely candidate for innovation, yet it is desirable, in this age of autonomous sensing where analyzers may be sent into space or to the bottom of the ocean, to have a simplified titrimetric method that does not rely upon volumetric or gravimetric measurement of sample and titrant. In previous work on the measurement of seawater alkalinity, we found that use of a tracer in the titrant eliminates the need to measure mass or volume. Here, we show the versatility of the method for diverse types of titrations and tracers. The results suggest that tracers may be employed in all types of titrations, opening the door for greatly simplified laboratory and field-based chemical analysis.     

Human drug self-administration in a medium of exchange.

In a laboratory setting, a medium of exchange procedure was used to study factors relevant to drug taking. Participants could earn an income, respond to obtain cigarette puffs, and purchase puffs. Across sessions, either the work required for money increased while the work required for puffs remained constant, or vice versa (puffs always cost $0.25 each). Each manipulation was conducted twice, once when they could keep income not spent on puffs, and once when they could not. The two manipulations had equivalent effects on drug taking when income could be spent only on puffs; both manipulations decreased drug taking in equal magnitudes. When participants could keep income not spent on puffs, however, the two manipulations had different effects. Participants spent less on puffs and consumed less when it was the work required for puffs that increased. These and other results show the utility of the medium-of-exchange procedure and the unit-price metric for evaluating basic features of the drug-taking environment. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Comparison between two detection systems for fiber-optic chemical sensor applications

Two optical detection system designs are compared for fiber-optic chemical sensor applications. A single grating spectrograph with fiber-optic input and photodiodes at three different wavelengths is compared to 1x3 fiber-optic splitters and photodiode detectors with integrated interference filters. The splitters are used to direct the optical power to the filter photodiodes. Three types of 1x3 commercially available splitters were tested: a 400 microm fused glass fiber-optic coupler, a 1000 microm fused plastic fiber-optic coupler, and a 1000 microm glass fiber-optic bundle. This study finds that the fiber-splitter-based detection systems have similar stray light, signal-to-noise ratio, and long-term absorbance stability compared to the spectrograph detection system with a modest improvement in spectral resolution (from approximately 12 nm to approximately 6 nm). It is also much smaller in size and lower in cost. Applications of the two systems in a colorimetric CO2 partial pressure sensor are compared and similar accuracy and precision are achieved.     

Effects of simulated employment and recreation on drug taking: A behavioral economic analysis.

With a within-subject design, this study examined the effects of varying response requirement (price) to obtain puffs on a cigarette in the presence and the absence of an opportunity (a) to make money in Experiment 1 (a simulation of employment) and (b) to engage in recreational activities (a simulation of recreation) in Experiment 2. The results indicated that cigarette smoking (number of puffs) decreased and response output increased as price increased. Also, cigarette smoking and response output decreased with the introduction of competing reinforcers at all prices. Lastly, the greatest reduction in cigarette smoking resulted from the combined effects of these two factors. Overall, these data suggest that drug use can be reduced by increasing price or enriching the environment and that maximal reductions result from the simultaneous application of both factors. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A science of meaning: Can behaviorism bring meaning to psychological science?

An argument is presented for making meaning a central dependent variable in psychological science. Principles of operant psychology are then interpreted as providing a basic foundation for a science of meaning. The emphasis here is on the generality of basic operant concepts, where learning is a process of meaning making that is governed largely by natural contingencies; reinforcement is an organic process in which environment–behavior relations are selected, defined here as dialectical process of meaning making; and reinforcers are experiential consequences with acquired, ecologically derived meanings. The author concludes with a call for a more interdisciplinary science of psychology, focusing on the individual in society. (PsycINFO Database Record (c) 2010 APA, all rights reserved)